The present invention relates to the enhancement of photographic images and in particular to the enhancement of commercial silver halide film utilizing a gold-toning neutron-activation process.
Contrast enhancement of photoreconnaissance and intelligence films is desirable since it allows the recovery of image information which may be taken under adverse lighting or obscured in the shadows and which might otherwise be lost by conventional methods using chemical/photographic or denistometer/computer techniques. Such a process can also be extremely valuable in the enhancement of weak images in other disciplines that use photographic film as detectors, e.g., low intensity spectroscopy, faint images in astronomy, and radiation dose reduction in diagnostic medical x-rays by reduction of required exposure.
When standard commercial silver halide film is underexposed and chemically developed with resulting atomic silver concentration of less than or equal to 10.sup.-5 gram/cm.sup.2, ordinary chemical enhancement and light transmission techniques are inadequate for satisfactory image formation. An image-silver density of 10.sup.-5 gram/cm.sup.2, however, can still contain a large amount of information (i.e., about 10.sup.7 silver-grains/cm.sup.2). Several nuclear and ion beam techniques can detect silver in light substrates in the 10.sup.-6 to 10.sup.-9 gram/cm.sup.2 range which is well below normal optical densitometer sensitivity. However, this sensitivity is not fully utilized because presently all contrast enhancement techniques, whether by nuclear or computer methods, reach a limiting condition imposed by the grain statistics and signal to noise ratio as the image density approaches the level of fog, i.e. the non-image fog which development produces an all sensitive films and which obscures images of lower density. Generally all of the present nuclear enhancement techniques employ some procedure for causing the silver grains in the photographic negative to emit charged particles or other ionizing radiation. The intensity of the induced activity is proportional to the density of the silver image. Both radiochemical toning and direct neutron activation have been used to make the silver grains emit electrons. In such techniques the radioactive image can be rendered visible and enhanced by placing the activated film in pressure contact with a new unexposed film which is exposed by the ionizing radiation rather than light (i.e., contact autoradiography). Thus the electrons from each grain in the original weak image can be made to cause numerous grains to be made developable in the autoradiograph thereby creating an amplification or enhancement mechanism.
The radiochemical toning techniques can be quite hazardous and require trained radiochemists. Typically the autoradiographic exposure times require seven to eighty hours. Direct neutron activation of the silver image, without toning, has several disadvantages. The principle difficulty is that the two silver isotopes which are produced by neutron activation have half-lives that are either inconveniently short (2.4 min.) for easy reproducible control of exposure or inconveniently long (252 days) for easy exposure. Furthermore, photographic materials such as gelatine are rapidly damaged by radiolysis and they can be left in a high flux reaction for only very short periods. While it is possible to achieve saturation activity of the short-lived isotope 108 Ag, only a small amount of .sup.110m Ag can be formed before the photograph is damaged. If time is no object, good autoradiographs can be obtained from this low level of .sup.110m Ag activity, but normally the short-lived isotope is used because of time restraints. In a technique utilizing the short-lived isotopes, irradiation of low density photographs at a thermal neutron flux of 7.times.10.sup.12 neutrons/cm.sup.2 /sec for 5 minutes followed by a cooling period of two minutes and autoradiographic exposure of 10 seconds has given dense autoradiographs, but the procedure is inconveniently hasty and estimation of the correct autoradiographic exposure is difficult.
Thackray has disclosed, in an Australian Pat. No. 422872, a silver iodide conversion process followed by neutron activation which partially eases some of the constraints of direct neutron activation of the silver. However, the process requires a forty minute neutron exposure and is effective for producing autoradiographs for only two or three hours due to the short twenty-five minute half-life of the .sup.128 I. (This generally means that the autoradiography must be carried out at the site of the reactor facility.) By contrast the gold-toning neutron-activation process of the present invention employs a stable chemically inert gold plating of the individual silver grains. Because of the unique neutron activation properties of gold, the subsequent neutron activation requires only five minutes and produces a radioisotope .sup.198 Au which has a 2.7 day half-life that allows autoradiography to be carried out for 7 to 10 days. Further the gold toning, being chemically inert, renders the image substantially impervious to environmental deterioration while leaving the visual tone quality unchanged and reusable. Moreover, the short thermal neutron activation period produces no detectable damage to the films.
Furthermore, instead of electron emission, the silver grains have also been made to emit heavy fission fragments by radiochemical toning with californium (spontaneous fission) and by uranium toning with fission by photon or neutron irradiation (induced fission). This technique requires a glass plate in pressure contact with the original toned film. Here, the fission fragments produce damage track in the glass plate. In order to render the image visible, the glass plate is developed in an etching solution. The fission track enhancement process is a considerably more involved technique requiring fairly precise control of etching and specialized printing. The degree of enhancement or amplification is not readily controllable since the etching process is irreversable. Further, the intense photon irradiation used, severely damages and alters the original film.